The present invention relates to a method of controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine, and more particularly, to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration.
Among conventional methods for feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (hereinafter referred to as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, there is a method proposed by Japanese Provisional Patent Publication (Kokai) No. 63-12850 wherein in changing the desired air-fuel ratio, the rate of change (speed of change) is changed over between a case in which the desired air-fuel stoichiometric air-fuel ratio (e.g. 14.7) and a case in which it is changed in an opposite direction (i.e. from a lean air-fuel patio to the stoichiometric air-fuel ratio).
The desired air-fuel ratio is changed to a lean value with respect to the stoichiometric value, for example, when the engine shifts to a predetermined partial load condition such as a so-called "lean burn" region to save the fuel consumption.
However, according to the proposed method, in changing the desired air-fuel ratio in the leaning direction from the stoichiometric air-fuel ratio, the rate of change is set to a constant value over the entire air-fuel ratio range on the lean side of the stoichiometric air-fuel ratio. Therefore, if the rate of change is set to a small value in order to prevent a torque shock from occurring due to a drastic change in the desired air-fuel ratio, there arises a problem that a time period is prolonged during which the desired air-fuel ratio assumes intermediate values, e.g. 16 and values in its vicinity an increase in the amount of emission of NO.sub.X. On the other hand, if the rate of change is set to a large value in order to curb the increase in the amount of emission of NO.sub.X, the torque shock increases, which results in degraded driveability.